1. Field of the Invention
The present invention relates generally to integrated circuit devices, and more specifically to power field effect device structures.
2. Description of the Prior Art
Power field effect devices continue to become increasingly popular for driving motors and other types of inductive loads. As manufacturers become more capable of producing circuits which can incorporate power field effect devices into a single integrated circuit chip with other MOS circuitry, this combined design is becoming more common. It is now possible to provide a complete set of control circuitry in the same integrated circuit chip with one or more power devices. This allows single chip products to be used for, for example, driving DC and stepper motors.
It is known to those skilled in the art that the current through an inductive load, such as a motor coil, cannot be cut off instantaneously. This property can cause problems with single chips which combine power and control devices. This is illustrated in the circuit of FIG. 1, in which MOS transistors 12, 14 are driving one side of inductive load 16. Typically, these types of inductive loads are connected into an H-bridge configuration, but the remainder of the bridge is not shown in FIG. 1. Instead, remaining circuitry on the other side of inductive load 16 is represented by resistive load 18.
Diode 20 is an artifact of the fabrication process for forming transistor 14, and in fact plays an important role in current switching when in circuits of the type shown in FIG. 1. Assume that, for example, current I1 is flowing through transistor 12, through inductor 16 and through resistive load 18 to node 22. Transistor 12 is on and transistor 14 is off. If transistor 12 is turned off, with transistor 14 remaining off, the current through the inductive load 16 cannot change instantaneously. However, current can no longer be drawn through transistor 12 from the supply voltage Vcc. This causes a phenomenon known as below-ground current injection, which causes current I2 to flow from ground, through diode 20, and through the loads 16, 18 to node 22.
This current, sometimes called a recirculation current in certain motor/driver configurations, is well known in the art. When the power driver transistors 12, 14 are discrete devices, this current causes little or no problem. However, if the control circuitry 12, 14 is formed on the same semiconductor substrate as the power devices, the presence of this current can adversely affect operation of such control circuitry.
Various isolation structures and techniques have been implemented to protect the control circuitry from below ground current injection. However, the structures and approaches used until now have not provided the degree of isolation which is desired.
It would therefore be desirable to provide an integrated circuit structured which give improved isolation and protection from below ground current injection.